Can’t/Don’t want to install cleanmgr?

How to delete Windows.old without running Disk Cleanup.

From:

takeown /f c:\Windows.old\* /r /a /d y
cacls c:\Windows.old\*.* /t /grant administrators:f
rmdir /s /q c:\Windows.old

Disk Cleanup is still the better route as it seems to get the job done eventually, but this does work, and I’ve used it several times. However, sometimes I had to run cacls more than once. That was an odd bug.